A photographic element for color photography usually comprises three silver halide photosensitive units sensitive to blue, green and red light that are respectively associated with yellow, magenta and cyan dye-forming compounds. Particularly useful dye-forming compounds are color-forming couplers. With this type of material, it is well known that color reproduction is often imperfect because of unwanted absorption of the dyes formed from the couplers. Furthermore, as described hereinafter, the development of silver halide in one of the emulsion layers during processing may affect dye formation in an adjacent layer.
In elements for color photography having three units with incorporated couplers, the three units respectively sensitive to blue, green and red light should be protected from undesirable interactions during storage, exposure and development with a view to obtaining excellent color reproduction. In addition, the spectral absorption of the dye formed from each incorporated color-forming coupler should be located in an appropriate wavelength range. These are well-known conditions to form a satisfactory color image. However, it is also known that elements for color photography exhibit various defects related to the difficulty of meeting these requirements.
As previously mentioned, one of the defects relating to color image reproduction is that the spectral absorption characteristics of the subtractive color images obtained from color-forming couplers are not satisfactory; i.e., the light absorption of the image dyes is not confined to a desired region of the spectrum and extends to other regions of shorter or longer wavelengths. There can also be overlap in the sensitizations of the associated silver halide emulsions. These defects result in degradation of colors.
Another defect arises because, during color development of the three color image-forming emulsion layers, the development of an image in one of the layers may cause unwanted formation of color in an adjacent emulsion layer intended by definition to record another image. For example, the development of the magenta image of the green-sensitive layer may cause formation of cyan dye in the red-sensitive layer, but following the pattern of the magenta image. This defect results from the fact that the oxidation products of development of one of the layers may diffuse to an adjacent layer where they would give rise to an unwanted coupling with the coupler present in this layer.
The above-mentioned defects cause what is sometimes referred to by the term "color contamination." The reaction for forming a dye image in a given emulsion layer affects the adjacent emulsion layers whereby the latter lose their aptitude to form independent elementary images and causes in these layers the formation of unwanted dye images by color contamination.
Because the problem has been acknowledged for a long time, various means have been recommended in the prior art to reduce or eliminate these color-contamination defects. For example, it has been proposed to incorporate in color image-forming photographic materials intermediate layers, or filter layers, comprising reducing compounds such as a hydroquinone or a phenol derivative, a scavenger for oxidized color-developing agent, couplers forming colorless compounds, or colored couplers forming diffusible dyes. However, none of these methods has been completely satisfactory.
Another method employs a development inhibitor-releasing, or DIR coupler, as described by Barr, Thirtle and Vittum in Photog. Sci. and Eng., Vol. 13, pages 74-80 and 214-217 (1969), and in U.S. Pat. No. 3,227,554. Generally, the DIR coupler releases in a layer an inhibitor pattern in accordance with the image formed in this layer, but which migrates into an adjacent layer, as described, for example, in U.S. Pat. Nos. 3,990,899 and 4,273,861. Thus, the DIR coupler provides a correction effect usually designated as an interlayer interimage effect. Such an effect may be accompanied by a strong intralayer inhibiting effect on development that necessitates a substantial increase in silver coverage. Because the DIR coupler has a limiting effect on development, the use of such a coupler can reduce contrast and maximum density.
Another method consists in changing the composition of the halides used in each layer respectively sensitive to blue, green and red light of the color photographic material by adjusting, for example, the proportion of iodide ions used in relation to bromide ions. This correction method is that traditionally used for color reversal photographic materials, and consists in causing an interimage effect during the first black-and-white development by the action of the iodide ions released from the developing silver haloiodide emulsions. In this method, however, the emulsion layers containing iodide ions are both causing and receiving interim age effects, so control of this effect can be difficult.
The very multiplicity of correction methods implies that none of them has been fully satisfactory. This is also true for other methods, known to have an influence on color correction, which entail variations in amounts of developing agents, sulfite ions, hydrogen ions, or buffering agents.
Positive dye image-forming reversal photographic materials have features different from those of negative dye image-forming photographic materials. For example, color reversal films have higher contrasts and shorter exposure latitudes than color negative film. Gammas for reversal films are generally between 1.5 and 2.0, which are substantially higher than those of negative films. Negative materials are processed, after image exposure, directly with a chromogenic developer that color develops the negative exposed areas. On the other hand, reversal materials, after imagewise exposure, are first processed with a black-and-white developer that develops a silver image in the negative exposed areas. This is followed by a reversal fogging step, a second overall exposure or a chemical fogging step, and then development with a chromogenic developer to form a positive color image.
In negative dye image-forming photographic materials, interim age effects are always obtained during chromogenic development. In positive dye image-forming reversal photographic materials, interimage effects are generally obtained, as mentioned above, during processing by the release in the first black-and-white developer of a development inhibitor as a function of the silver development of the image-forming layers. The most generally used development inhibitor consists of iodide ions released as a result of the development of silver haloiodide, for example, silver bromoiodide emulsions. EP Application No. 442323, for example, discloses a color photographic reversal material whose total light-sensitive silver halide grains have an average silver iodide content of about 5.5 mole percent or less and a pair of light-sensitive silver halide emulsion layers having differing color sensitivity and a difference of at least 1 mole percent in average silver iodide content, and which has as an object the reproducibility of shades of colors in high density areas.
To obtain interimage effects in dye image-forming reversal photographic materials, the formation of interimage effects in the second chromogenic developer by development inhibitors, such as iodide ions or mercaptans released from incorporated DIR couplers, has generally been avoided because poor results have been obtained. For example, if a DIR coupler is incorporated in a dye image-forming layer of a reversal photographic material, increased granularity of the color positive image may result.
When DIR compounds are proposed for use in color reversal materials, it has been suggested that color development be limited, for example, by reducing development time. It has also been proposed in U.S. Pat. Nos. 4,729,943 and 5,051,345 and in European Patent Application No. 296,784 that, for purposes of improved color reproducibility, a DIR compound be utilized in a layer that contains a silver halide emulsion but does not contribute to image formation. The use of DIR compounds with specific types of couplers, for example, pyrazoloazole magenta couplers in EP Application No. 296,785, has also been proposed.
All of these suggestions of prior workers have serious drawbacks. For example, any technique that employs an extra silver halide emulsion layer has some obvious drawbacks. Silver halide use is increased, which adds to the cost of production and to the cost of film processing. Moreover, addition of an additional layer adds to film thickness, and this increases light scattering during exposure. Light scattering decreases image sharpness, and thus an increase in film thickness is not desired in color reversal film technology.
This invention can be used to overcome the disadvantages discussed above. Furthermore, a very significant advantage of this invention is that it allows use of standard processes such as the Kodak E-6 development process without modification. That process provides the advantages inherent in using all, or nearly all, of the exposed silver to form the image obtained from the exposed film. The E-6 process is commonly employed today; it and substantially equivalent processes made available by other manufacturers are so widely used that films are designed to be satisfactorily developed by these processes. In most instances the E-6 process, or a substantially equivalent process, is the only reversal process used by a business entity that develops reversal film. Accordingly, this invention has inherent advantages over any prior art suggestion that necessarily involves the use of a modified color reversal process.
Moreover, any previously proposed use of DIR compounds in color reversal systems that requires the use of a specific type of magenta coupler, severely limits the proposed system by making it less than generally applicable. This invention, which does not require specific types of couplers, has broad applicability.